Patterns for Refinement Automation - Formal Methods for Components and Objects - page 95

of channels simultaneously by interleaving events associated with separate
channels.
4.1 Requirements for Establishing a Media Channel
Either end of a channel, sender or receiver, can attempt to open a media channel
by sending an open signal. The other end can respond armatively with openAck
(Open Acknowledge) or negatively with close . A media flow can be established
between two media endpoints if and only if both media endpoints agree.
Each open signal carries the medium being requested, and a descriptor. A
descriptor is a record in which an endpoint describes itself as a receiver of media.
A descriptor contains an IP address, port number, and priority-ordered list of
codecs that it can handle. If the endpoint does not wish to receive media, then
the only offered codec is noMedia .Each openAck signal also carries a descriptor,
describing the channel acceptor as a receiver of media.
A selector is a response to a descriptor. A selector is a record in which an
endpoint describes itself as a sender of media. It contains the identification of
the descriptor it is responding to, the IP address of the sender, and the port
number of the sender. If the selecting endpoint does not wish to send media,
then the selector contains noMedia ; otherwise, it contains a single codec selected
from the list in the descriptor. The only legal response to a descriptor noMedia
is a selector noMedia .
After sending an open signal by the initiator side of the channel, and sending
an openAck signal by the other side, called the acceptor , both endpoints have
to respond to descriptors carried by the open and openAck signal, by sending a
select signal carrying a selector. As said before, it is a rule of the protocol that
a selector should be sent in order to respond to receiving a descriptor. A media
channel is established by the endpoint, initiator or acceptor, which receives a real
codec in a select signal. Fig. 6 illustrate a life cycle of a media channel starting
with establishing the channel.
4.2 Requirements for Modifying an Established Media Channel
Modifying an established media channel may involve changing the codec and
changing the port of each endpoint. At any time after sending the first selector
in response to a descriptor, an endpoint can choose a new codec from the list in
the descriptor, send it as a selector in a select signal, and begin to send media
in the new codec. In Fig. 6, select(sel'2) shows this possibility.
At any time after sending or receiving oAck , an endpoint can send a new
descriptor for itself in a describe signal. The endpoint that receives the new
descriptor must begin to act according to the new descriptor. This might mean
sending to a new address or choosing a new codec. In any case, the receiver of the
descriptor must respond with a new selector in a select signal, if only to show
that it has received the descriptor. In Fig. 6, describe(desc3) and select(sel3)
illustrate this interaction. Finally at any time after sending or receiving oAck ,
an endpoint can send a new port and describe itself by a new port.